The overall goal of this research program is to understand transplantation tolerance mediated by costimulation blockade. This form of transplantation tolerance is associated with the deletion of alloreactive CDS T cells. Importantly, the activation of innate immunity by virus infection or exposure to Toll-like receptor agonists can prevent both alloreactive CDS T cell deletion and tolerance induction. The specific focus of this Study is to define the molecular mechanisms of CDS T cell apoptosis. We propose to examine the biochemical mechanism of CDS T cell death (Specific Aim 1). These Studies will provide the foundation for molecular studies of specific pathways of CD8 T cell death (Specific Aims 2 & 3). It is established that members of the Bcl2 protein family act as critical regulators of CDS T cell death. Moreover, members ofthe stress-activated protein kinase family are implicated in the regulation of CDS T cell death. We will examine these pathways during the induction of transplantation tolerance during co-stimulation blockade. We will also examine CDB T cell death when transplantation tolerance is disrupted by exposure to Toll-like receptor agonists and lymphocytic choriomeningitis virus (LCMV) infection. These studies are fully integrated within the theme of the Program Project and depend upon collaborative studies with the other Projects. The Specific Aims of this proposal are to examine the: 1. Biochemical mechanism of CDS T cell death. 2. Role of stress-activated MAP kinases in CDS T cell death. 3. Role of Bcl2 family proteins in CDB T cell death. We anticipate that the successful completion of these studies will provide important new insight into the understanding of transplantation tolerance and that the new information we obtain will contribute to the design of therapies for the treatment of human disease.